While a turbojet is in operation, the casings are subjected to vibration that may lead to damage in the long run. By way of example, a low pressure compressor casing includes a flange that serves in particular to have equipment fastened thereto and that includes numerous holes for passing bolts.
The mechanical stresses in the vicinity of these holes, caused by the vibration of the casing, give rise to damage by hammering of the inside surfaces of the holes and of the plane surface of the flange against which the equipment or the equipment support bears. Localized wear is thus observed in the surface of the flange that comes into contact with the equipment or the equipment support, accompanied by an increase in the diameter of the hole and ovalization of the section of the hole.
In order to ensure proper operation of the turbomachine, it is necessary either to repair this damage, or else to change the casing completely, it being understood that the average cost thereof is very high.
In order to repair such damage, it is known to add material by welding so as to reconstruct the original shape of the holes and the surface of the flange against which the equipment or the equipment support comes to bear.
Nevertheless, when casings are made of aluminum or of composite material, and more generally out of any non-weldable material, such a method cannot be used. As an alternative, material may be applied using a fiberglass-filled epoxy resin. That technique is used to restore the original shape to holes, but it cannot be used for repairing the damaged zone of the surface of the flange against which the equipment bears. The compression strength of such resin is not sufficient to guarantee a rigid interface with the equipment assembled on the flange. Furthermore, when the bolts are tightened, only the undamaged portion of the flange is capable of withstanding the compression forces. Since the undamaged bearing surface is reduced, the hammering and wear pressure on the flange are increased. Thus, even after holes have been repaired, it might still be necessary to change the casing, as a result of excessive wear of said flange surface.
Furthermore, such a repair cannot be performed during an under-the-wing maintenance operation, but on the contrary it requires the entire engine to be removed.
It should also be observed that regulations ban the fitting of additional parts to a certified configuration.
Patent application FR 10/00555 in the name of the Applicant and not yet published describes a method of repair that consists in machining a spot face in the worn zone of the flange and then adhesively bonding a washer on the spot face.
Nevertheless, such a spot face and such a washer are not suitable for all types of flange. In particular, depending on the shape of the flange, the diameter of the washer sometimes needs to be small, thereby limiting the strength of the resulting interface with the equipment or the equipment support.